1. Field of the Invention
The present invention relates to an emissive type flat panel display device, and more particularly, to an organic light emitting display with a via hole that is larger than a contact hole in a structure where the via hole is formed corresponding to the contact hole.
2. Discussion of the Background
Generally, in an organic light emitting display (OLED), which is a flat panel display device, the size of cells may be reduced to increase resolution. Accordingly, sizes of a contact hole, which connects source/drain electrodes of a thin film transistor (TFT) to source/drain regions, and a via hole, which connects a TFT's source or drain electrode to a lower electrode of an organic electroluminescence device, may be reduced.
However, when the size of the via hole is reduced, a pattern defect may be generated when patterning the lower electrode, thereby decreasing the device's reliability. In a structure where the via hole is formed corresponding to the contact hole, the probability of generating the pattern defect may increase due to a step that is generated when forming the contact hole.
FIG. 1 is a cross-sectional view showing a conventional OLED.
Referring to FIG. 1, a buffer layer 105 is formed on an insulating substrate 100, and a semiconductor layer 110, including a source region 111 and a drain region 115, is formed on the buffer layer 105. A gate electrode 125, which corresponds to a channel region 117 of the semiconductor layer 110, is formed on a gate insulating layer 120. A source electrode 141 and a drain electrode 145 are electrically connected with the source and drain regions 111 and 115 through contact holes 131 and 135, respectively. The contact holes 131 and 135 are formed in an interlayer dielectric 130 and the gate insulating layer 120.
Additionally, a protective layer 150 and an overcoat 160 are sequentially formed on the interlayer dielectric 130. The protective layer 150 includes a first opening portion 155 that exposes a part of the drain electrode 145, and the overcoat 160 includes a second opening portion 165 that also exposes a part of the drain electrode 145.
A lower electrode 170 may be formed on the overcoat 160, and it may be electrically connected to the drain electrode 145 through a via hole 167. The lower electrode 170 is a pixel electrode, and it may include a metal reflective layer 171 and a transparent conductive layer 175. The reflective layer 171 may include a third opening portion 177 that exposes a part of the drain electrode 145.
A pixel defining layer 180, which is formed on the overcoat 160, includes a fourth opening portion 185 that exposes a part of the lower electrode 170. An organic layer 190, including a light emitting layer, is formed on the lower electrode 170 in the fourth opening portion 185, and an upper electrode 195 is formed on the insulating substrate 100.
FIG. 2A is a cross-sectional view, taken along line IIA-IIA of FIG. 2B, showing a part of the OLED of FIG. 1 corresponding to the contact hole and the via hole, and FIG. 2B is a plan view showing the contact hole and the via hole in the OLED of FIG. 1.
Referring to FIG. 2A and FIG. 2B, the conventional OLED may include the circular-shaped contact hole 135 and via hole 167, and the plan view area of the via hole 167 is contained within the plan view area of the contact hole 135. The contact hole 135 exposes the drain region 115 of the semiconductor layer 110 for connecting to the drain electrode 145, and it is formed in a gate insulating layer 120 and the interlayer dielectric 130 to have a size d11. The via hole 167 exposes the drain electrode 145 for connecting the drain electrode 145 to the lower electrode 170, and it includes the first opening portion 155, which is formed in the protective layer 150 and has a size d12, and the second opening portion 165, which is formed in the overcoat 160 and has a size d13.
Here, the size d11 of the contact hole 135 is a cross-sectional length of the drain region 115 that is exposed by the contact hole 135, and the size d12 of the first opening portion 155 is a cross-sectional length of the drain electrode 145 that is exposed by the first opening portion 155. Further, the size d13 of the second opening portion 165 is a cross-sectional length of the drain electrode 145 that is exposed by the second opening portion 165. Additionally, the size of the via hole 167 is a cross-sectional length of the drain electrode 145 that is exposed by the via hole 167. Thus, the second opening portion 165 has a smaller size d13 than the size d12 of the first opening portion 155, and the protective layer 150 is covered by the overcoat 160. Therefore, the size of the via hole 167 is determined by the size d13 of the second opening portion 165.
In a conventional front surface emissive type OLED, the lower electrode 170 may serve as the anode electrode, and it may have a stacked structure including a reflective layer 171, such as an AlNd layer, and a transparent conductive layer 175. However, when using an Al alloy, such as AlNd, as the lower electrode, an oxide layer, such as Al3O3, may form on an interface between the drain electrode 145 and the lower electrode 170, thus increasing contact resistance thereon.
In order to reduce the contact resistance, the reflective layer 171 may be patterned so that the transparent conductive layer 175 may be formed directly on the drain electrode 145 in the via hole 167. That is, the reflective layer 171 is deposited on the insulating substrate 100, a photoresist layer (not shown) is applied on the reflective layer 171, and a photolithography process is performed to pattern the photoresist. Here, the photoresist layer located on the portion of the insulating substrate 100 corresponding to the via hole 167 is patterned to be removed. Additionally, the reflective layer 171 is patterned to have the third opening portion 177 that exposes the drain electrode 145 in the via hole 167 using the patterned photoresist layer.
Removing the portion of the reflective layer 171 contacting the drain electrode 145 may solve the contact resistance problem. However, in the front surface emissive type OLED, in which the via hole 167 is formed corresponding to the contact hole 135 and the overcoat 160 is formed below the lower electrode 170, the via hole 167 may be formed in the overcoat 160 smaller than the contact hole 135. Thus, a depth of the via hole 167 increases and a diameter of the via hole 167 gradually decreases, thus the photoresist layer in the via hole 167 may not be completely exposed when patterning the photoresist.
Therefore, photoresist may remain in the via hole 167 after patterning, and the reflective layer 171 may not be removed from the via hole 167 due to the remaining photoresist when the reflective layer 171 is subsequently patterned. Therefore, this may cause a pattern defect of the lower electrode 170. Additionally, when the via hole 167 has a small size, the contact between the drain electrode 145 and the lower electrode 170 may be defective, thereby increasing the contact resistance.